Show captionA newly discovered world dwarfs our own largest planet, Jupiter pictured above, and could be a kind of ‘failed’ star known as a brown dwarf. Adding to the mystery, it has been found in a forbidden region around its star called the brown dwarf desert. Photograph: NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles

‘When is a planet not a planet?’ is a lot more than the beginning of a poor joke at a drunken astronomers’ Christmas party (but we laughed nonetheless). It is actually a serious question that cuts to the heart of our ignorance about how celestial objects form.

The discovery of a giant planet 22,000 light years away may now help shine some light on this particularly knotty problem. The planet is called OGLE-2016-BLG-1190. It was found on June 2016 by the Optical Gravitational Lensing Experiment (Ogle), a Polish astronomical project run by the University of Warsaw.

The planet was not seen directly but was inferred by the way its gravity focused light from its parent star, causing the star to temporarily brighten. Such a phenomenon is known as a gravitational microlensing event. It was observed by a number of different observations besides Ogle, one of which was the orbiting Nasa space telescope Spitzer.

In a study just published by Y-H Ryu of the Korea Astronomy and Space Science Institute, and colleagues, the astronomers calculate that the planet must be somewhere around 13 times the mass of Jupiter, the largest planet in our solar system.

Jupiter itself is 317 times more massive that the Earth, making this newly discovered world around 4000 times the mass of our planet. This puts it right on the boundary between a planet and a type of “failed” star called a brown dwarf.

Astronomers have a number of possible ways that they can decide whether something is a star or a planet. The most obvious is that a planet does not generate significant energy whereas a star does. This is why a star shines but planets just reflect light. A brown dwarfs sits between the two.

It is not a star because it does not fuse hydrogen to generate significant energy and so does not shine brightly. It is not a planet either, because it can fuse an isotope of hydrogen called deuterium, thus generating a small amount of energy.

At 13 Jupiter masses, this newly discovered objects sits right at the boundary between planet and brown dwarf. But that’s not the most interesting thing. The thing that is really opening eyes is where the behemoth is located.

It orbits its star at about twice the distance of Earth from the sun, placing it smack in the middle of the so-called brown dwarf desert.

Ever since the first planets around other stars were found in 1995, astronomers have been surprised by the lack of brown dwarfs in orbits around other stars closer than five times the Earth’s distance from the sun. This seemingly forbidden zone is what they have termed the brown dwarf desert.

One possible reason for it is based on how brown dwarfs might form. Planets grow from the accumulation of smaller, asteroid-sized chunks of rock and metal. Stars, and possibly also brown dwarfs, form from the collapse of a gas cloud. It is unlikely that there would be enough gas close to a star to collapse and form a brown dwarf because the gas would be driven to more distant, colder regions by the heat from the star.

But if that is true, how come OGLE-2016-BLG-1190 has turned up in the desert? Now, it’s over to the theoreticians and their computer models to see if they can figure out some possible scenarios for how this oddball formed. In the meantime, the hunt for more objects in the brown dwarf desert around other stars will begin.

As for OGLE-2016-BLG-1190 itself, the planet has been calculated to have an orbit of roughly three years. This means that astronomers may get another glimpse of it in 2019 when it should induce another microlensing event of its parent star.